Promising as inexpensive, solid-emission-type, large-emission-area, full-color display devices and writing light source arrays, the organic, light-emitting devices have been actively developed. The organic, light-emitting device generally comprises a couple of electrodes, and a light-emitting layer formed therebetween. When an electric field is applied to both electrodes, electrons are injected from the cathode, while holes are injected from the anode. When electrons and holes are recombined in the light-emitting layer, whereby an energy level is lowered from a conduction band to a valence band, energy is turned to light, which is emitted from the organic, light-emitting device.
The conventional organic, light-emitting devices are disadvantageous in that they require high driving voltage for light emission and are poor in luminance and light-emitting efficiency. Some proposals have recently been provided to overcome the above disadvantages. For example, a light-emitting device comprising organic thin layers formed by vapor-depositing organic compounds is disclosed in Applied Physics Letters, 51, 913 (1987). This organic, light-emitting device has a two-layer laminate structure comprising an electron-transporting layer and a hole-transporting layer, exhibiting largely improved light-emitting properties than those of conventional organic, light-emitting devices having a single-layer structure. This organic, light-emitting device uses a low-molecular-weight amine compound as a hole-transporting material and 8-quinolinol aluminum complex (Alq) as an electron-transporting, light-emitting material, emitting green light. After this disclosure, many organic, light-emitting devices comprising vapor-deposited organic thin layers have been reported, as disclosed in Macromolecularly Symposium, 125, 1 (1997) and references therein, etc.
For the purpose of production cost reduction and application to flexible large-area devices such as backlights and illumination light sources, organic, light-emitting devices formed from high-molecular-weight, light-emitting compounds by a wet film-forming method have also been reported. As the high-molecular-weight, light-emitting compounds, there are, for instance, poly(p-phenylenevinylene) generating green light (Nature, Vol. 347, page 539, 1990), poly(3-alkylthiophene) generating reddish orange light (The Japanese Journal of Applied Physics, Vol. 30, page L 1938, 1991), polyalkylfluorene generating blue light (The Japanese Journal of Applied Physics, Vol. 30, page L1941, 1991), etc. Also, JP 2-223188 A reports an attempt to disperse low-molecular-weight, light-emitting compounds in binder resins, and wet-coat the resultant dispersion to form films.
However, in any of light-emitting devices produced by the above dry method and those produced by the above wet method, the use of a flexible, plastic substrate provides extremely lower durability than the use of a glass substrate. Accordingly, it has been considered difficult to provide commercially acceptable light-emitting devices by the wet method. One of the reasons therefor is that a plastic substrate of PET, etc. has such large gas permeability and moisture permeability that the penetrating oxygen and moisture exert adverse effects on the performance of the light-emitting device. When moisture exists in the light-emitting device, current flowing therein electrolyzes the moisture, generating a hydrogen gas and an oxygen gas and thus resulting in dark spots. In addition, because extremely easily oxidizable metals are used for the cathode, they are reacted with moisture and oxygen, causing dark spots.
Another reason is that because there is difference in a linear thermal expansion coefficient by one order or more between the flexible, plastic substrate and the electrode materials (ITO and metals), electrode materials tend to peel off from the substrate due to a thermal hysteresis, resulting in cracking and decrease in the durability of the light-emitting device.
A further reason is that because the substrate should be substantially transparent for the reason that the light-emitting device has a basis structure of substrate/anode/organic compound layer/cathode, in which the emitted light is taken out from the anode substrate side, there has not been developed yet a flexible support substrate that is transparent and has as high barrier properties to moisture and oxygen as those of glass. For instance, JP 2001-185348 A proposes a sealing layer comprising an insulating layer laminated with a metal layer to have high barrier properties to moisture and oxygen. However, this is a sealing layer that cannot be used as a substrate. Though JP 2001-60495 A and JP 11-320744 A describe the linear thermal expansion coefficients of barrier layers and protective films, they fail to describe the linear thermal expansion coefficients of flexible support substrates.